1. Field of the Invention
The present invention relates to a semiconductor device, in particular,
2. Background Arts
A high electron mobility transistor (HEMT) including nitride semiconductor materials, in particular gallium nitride (GaN), has become popular as a semiconductor device having a high breakdown voltage and showing high output power. For instance, a Japanese Patent Application laid open No. 2005-086102A has disclosed a HEMT providing a buffer layer, a lower barrier layer, a channel layer, an upper barrier layer, and a cap layer sequentially stacked on an insulating substrate in this order.
The cap layer in a thickness thereof may control or adjust a threshold voltage of a HEMT. However, the cap layer in a thickness thereof also affects long-term stability of performance of the HEMT. That is, the threshold voltage of a HEMT often shows a positive shift as an operating time thereof increases, which decreases a drain current when the HEMT is constantly biased.